Steroid hormones, including estrogens, play an essential role in metabolism, sexual differentiation, and reproductive function. Considerable attention, therefore, has been directed to defining the mechanisms that control their biosynthesis. We have been studying the mechanisms that regulate the expression of the human aromatase gene in breast cancer. Aromatase catalyzes the conversion of androgens to estrogens and plays a key role in the pathogenesis of hormone-dependent breast cancer. An understanding of the mechanisms that regulate expression of the human aromatase gene will allow one to develop assays to screen for drugs for treating or preventing breast cancer, e.g., by screening for drugs which positively or negatively modulate transcription of the aromatase gene. Research from our laboratory has identified a silencer element (Wang and Chen, 1992; Zhou and Chen, 1998) that is situated between two aromatase promoters, 1.3 (Zhou et al., 1996a) and II (Wang and Chen, 1992; Zhou and Chen, 1998), which are thought to be the major promoters controlling aromatase expression in the ovary and in breast cancer tissue (Zhou et al., 1996b; Harada, 1997; Agarwal et al., 1996). UV cross-linking experiments (Wang and Chen, 1992; Zhou and Chen, 1998) have found that at least four proteins bind to the silencer element. Two orphan nuclear receptors, SF1 (Steroidogenic factor 1) and ERRα1 (Estrogen related receptor α-1), were shown to bind to this regulatory region (Yang et al., 1998). Cell transfection experiments have revealed that both SF1 and ERRα1 function as positive regulatory factors when they bind to the silencer element (Yang et al., 1998).
Nuclear receptors are transcription factors that modulate transcription of various cellular genes, either positively or negatively, by interacting with specific hormone-responsive elements located in the target gene promoters and thereby control diverse aspects of cell growth, development, and homeostasis. The mechanisms by which the nuclear receptors can regulate the transcription from the target gene promoters are currently under intensive investigation. Recent data show that, in addition to contacting the basal transcriptional machinery directly, nuclear receptors enhance or inhibit transcription by recruiting an array of coactivator and corepressor proteins to the transcription complex. Recently, a number of these putative co-regulatory proteins for nuclear receptors have been identified, and have been shown to act either as coactivators or as corepressors (reviewed in Horwitz et al., 1996; Shibata et al., 1997; Glass et al., 1997). Among the members of a growing family of coactivators are CBP and members of the SRC-1 gene family including SRC-1/pl60 (Onate et al., 1995; Halachmi et al., 1994; Kamei et al., 1996), TIF2/GRIP-1 (Voegel et al., 1996; Hong et al., 1996; Ding et al., 1998), and CBP/p300 (Chakravarti et al., 1996; Hanstein et al., 1996) which function as coactivators of nuclear receptors, and also RIP140 (Cavailles et al., 1994; Cavailles et al., 1995), TIF1 (Le Douarin et al., 1995) and TRIP1/SUG-1 (Lee et al., 1995; vom Baur et al., 1996), the functions of which are not clearly defined. Most of these cofactors of nuclear receptors have a molecular weight around 160 kDa, and share a common motif containing a core consensus sequence LXXLL (L, leucine; X, any amino acid), which is necessary and sufficient to mediate the binding of these proteins to liganded nuclear receptors. LXXLL is thus a defining feature of pl60 coactivators (Heery et al., 1997).
Although these cofactors are of interest, additional co-activators and co-repressors are sought, especially those which interact with receptors via a novel binding motif.
The publications and other materials used herein to illuminate the background of the invention or provide additional details respecting the practice are incorporated by reference and for convenience are respectively grouped in the appended List of References.